Human immunodeficiency virus (HIV) causes a variety of clinical manifestations such as opportunistic infection, Kaposi's sarcoma, wasting syndrome, and neurological defects. The main goal of this project is to determine the role of variation in viral sequences in causing these different syndromes. Studies this year have mainly focussed on the role of the V3 region of envelope in influencing HIV-1 infection of macrophages. Two nonmacrophage-tropic patient derived clones were compared to a prototype macrophage-tropic clone. The results indicated that specific amino acids at five V3 positions influenced macrophage tropism, whereas amino acids at three to four different positions influenced syncytia induction(SI or NSI phenotype) in lymphocytes. These experiments revealed that macrophage tropism and the NSI phenotype were different phenotypes controlled independently by different viral sequences rather than two phenotypic representations of the same viral gene function. Additional studies have also involved in vitro infection of human macrophages by HIV strains which vary in replication levels in macrophages. This variation was found to occur only in macrophages and not in lymphocytes, and it correlated with sequence differences in the envelope V1 and V2 regions. Immunostaining of virus-positive cells at various times after infection showed that high replication levels were due to viral spread in the macrophages whereas low replication levels were seen in viruses which were unable to spread to new cells after initial infection. These two replication phenotypes have been seen in primary AIDS patient HIV isolates and might play different roles in pathogenesis in vivo. Further experiments have been carried out studying the role of HIV-1 variation on influencing tropism for human brain capillary endothelial cells. Results so far suggest that a region at the N-terminus of the env gene is important for this cell tropism.